There are profound effects of vitamin D on immune function. There is data to suggest that increased levels of vitamin D (either through diet, supplements or sunlight exposure) are beneficial for multiple sclerosis (MS) patients. In addition, active vitamin D (1,25(OH)2D3) treatment of experimental autoimmune encephalomyelitis (EAE) blocks the development of disease. In the first year of life infants undergo considerable fluctuation in their circulating vitamin D (25(OH)D3) levels. We hypothesize that early changes in vitamin D have profound effects on the thymus such that iNKT cells fail to develop and as a result autoimmune diseases like MS are more likely to develop. iNKT cells require adequate vitamin D in utero and the expression of the vitamin D receptor (VDR) for both development and function. iNKT cells have been implicated as inhibitors and regulatory cells in EAE and MS. The hypothesis to be tested is that vitamin D regulates iNKT cells and as a result is important for shaping the developing immune response and preventing and controlling the development of autoimmunity. The goals of this proposal are to determine the molecular mechanisms underlying vitamin D and the VDRs effects on iNKT cells. The aims are to determine the plasticity of the iNKT cell response as a function of changes in vitamin D status, determine the role of vitamin D in the thymus for iNKT cell development, determine how changes in vitamin D affect proliferation, survival and death of iNKT cells, and to determine the role of active vitamin D (1,25(OH)2D3) regulation of iNKT cells in suppression of EAE and whether changes in vitamin D status in vivo or 1,25(OH) 2D3 in vitro regulates human NKT cell expansion and function. NKT cells are novel targets in MS and a better understanding of the mechanisms by which their development and function are regulated by vitamin D and 1,25(OH)2D3 would be critical for manipulating NKT cells therapeutically.